I would difinitely use Ctek battery charger and leave it connected to the car to charge the battery if not using for a couple of days. It will even re condition your battery and desulphate it.

If the battery is 3 years old and this is the first time he's gotten a low voltage indicator, it's pretty unlikely that there's any sulfation.

Basically, the car's power system is super complicated. It uses lots of power when the car isn't running, and as a result, does a lot of power monitoring and consumption management. If you don't drive the car all that frequently, it won't charge up the battery, and thus it'll be pretty easy for you to drain it by, for example, accidentally leaving a light on.

That said, you won't damage the battery unless you deep discharge it multiple times.

Bottom line, if you get a low battery indicator, drive the car to charge it up. If you get one frequently, get a battery tender as people suggest.